On-p-GaAs substrate Zn1-xMgxSySe1-y pin photodiode and on-p-GaAs substrate Zn1-xMgxSySe1-y avalanche photodiode

ABSTRACT

A blue-ultraviolet on-p-GaAs substrate pin Zn 1-x Mg x S y Se 1-y  photodiode with high quantum efficiency, small dark current, high reliability and a long lifetime. The ZnMgSSe photodiode has a metallic p-electrode, a p-GaAs single crystal substrate, a p-(ZnSe/ZnTe) m  superlattice (m: integer number of sets of thin films), an optionally formed p-ZnSe buffer layer, a p-Zn 1-x Mg x S y Se 1-y  layer, an i-Zn 1-x Mg x S y Se 1-y  layer, an n-Zn 1-x Mg x S y Se 1-y  layer, an n-electrode and an optionally provided antireflection film. Incidence light arrives at the i-layer without passing ZnTe layers. Since the incidence light is not absorbed by ZnTe layers, high quantum efficiency and high sensitivity are obtained.  
     A blue-ultraviolet on-p-GaAs substrate avalanche Zn 1-x Mg x S y Se 1-y  photodiode with high sensitivity, high quantum efficiency, a wide sensitivity range, high reliability and a long lifetime. The ZnMgSSe avalanche photodiode has a metallic p-electrode, a p-GaAs single crystal substrate, a p-(ZnSe/ZnTe) m  superlattice (m: integer number of sets of thin films), an optionally formed p-ZnSe buffer layer, a p-Zn 1-x Mg x S y Se 1-y  layer, a lower doped n − -Zn 1-x Mg x S y Se 1-y  layer, a higher doped n + -Zn 1-x Mg x S y Se 1-y  layer, an n-electrode and an optionally provided antireflection film. Since the incidence light is not absorbed by ZnTe layers, a high avalanche gain, high quantum efficiency and high sensitivity are obtained.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to ZnSe type pin and avalanche photodiodes havinga sensitivity range from ultraviolet rays to blue light. A pinphotodiode (pin-PD or PD) is an optoelectronic device having functionsof making a strong electric field at a pn junction by applying reverse.bias, receiving input light, generating pairs of holes and electrons,accelerating the holes and electrons by the reverse bias and producingphotocurrent by the holes and electrons in proportion to power of theinput light. An avalanche photodiode (APD) further amplifies flows ofcarries (electrons and holes) by inducing an avalanche of carriers by apre-applied large reverse bias.

This application claims the priority of Japanese Patent Application No.2002-244795 filed on Aug. 26, 2002, which is incorporated herein byreference.

A photodiode has sensitivity for light of a bandgap wavelength λgcorresponding to a bandgap Eg of a material of a light receiving layer(active layer) and for light of wavelengths shorter than the bandgapwavelength λg. Here, the bandgap wavelength is defined by an equation ofλg=hc/Eg (h is Planck's constant, c is light velocity in vacuum). Aphotodiode cannot detect light of wavelengths far shorter than thebandgap wavelength λg. Different kinds of photodiodes should be utilizedfor different wavelength bands of object light. Most prevalentphotodiodes are silicon photodiodes (Si-PD) at present. Silicon has abandgap Eg=1.1 eV and a bandgap wavelength λg=1.1 μm. Siliconphotodiodes (Si-PDs) have a wide sensitivity range from visible tonear-infrared light (λ≦1.1 μm). Germanium photodiodes (Ge-PDs) areemployed for detecting light of wavelengths longer than the sensitivityrange of Si-PDs. Germanium (Ge) has a bandgap Eg=0.67 eV and a bandgapwavelength λg=1.8 μm. Ge-PDs can detect infrared light of wavelengths upto 1600 nm. Si-PDs and Ge-PDs can cover a far wide wavelength range fromvisible light to infrared light. Besides Si-PDs and Ge-PDs, indiumphosphide photodiodes (InP-PDs) which have an InGaAs sensing layer piledupon an InP substrate are utilized for sensing light of a 1.55 μm bandand a 1.3 μm band prevailing in optical communications. The three typesof photodiodes—Si-PDs, Ge-PDs and InP-PDs—are prevalent. But, thesethree kinds of photodiodes cover only the wavelength range of thevisible light and infrared light. None of the three types of photodiodeshas sensitivity for blue, violet and ultraviolet rays. No photodiodesare still available for detecting light of shorter wavelengths. Evensilicon photodiodes have poor sensitivity for violet andnear-ultraviolet rays.

Reception of violet light and ultraviolet rays requires new photodiodeshaving an active layer made of a material with a wide bandgapcorresponding to object light colors of short wavelengths, blue, violetor ultraviolet.

Gallium nitride (GaN) and zinc selenide (ZnSe) are well known as a widebandgap material. Gallium nitride (GaN) is an excellent material forblue-light emitting devices (light emitting diodes (LED) and laserdiodes (LDs)). GaN has overcome ZnSe as a material for light emittingdevices. Gallium nitride (GaN), however, is bad for a material ofphotodetectors. No good GaN substrate single crystal is obtainable yetat present. If we tried to make a GaN photodiode, we wouldheteroepitaxially pile GaN-layers on a sapphire substrate. GaN layersgrown on a sapphire substrate have many dislocations and defects. Anon-sapphire GaN photodiode would be annoyed at poor sensitivity andlarge dark current due to large defect density.

Zinc selenide (ZnSe) was defeated by GaN in the race of making lightproducing devices (LEDs or LDs). ZnSe is still promising as a materialfor making light receiving devices (PDs or APDs) instead of lightproducing devices. ZnSe has a bandgap wavelength λg=460 nm. λg=460 nmgives ZnSe a possibility of becoming a favorable material forphotodiodes for detecting blue light and violet light.

One purpose of the present invention is to provide a ZnSe typephotodiode enabling us to detect blue, violet and ultraviolet rays.

2. Description of Related Art

At present, materials which enable us to obtain large single crystalsubstrates are silicon (Si), germanium (Ge), indium phosphide (InP),gallium arsenide (GaAs) and gallium phosphide (GaP). No large good bulksingle crystal of zinc selenide can be grown yet. ZnSe type devicesshould be built on foreign material substrates. N-type gallium arsenide(GaAs) single crystals have been used as substrates for making ZnSe typedevices, since the lattice constant of ZnSe is close to that of GaAs.

Electrons have far higher mobility than holes in GaAs. In general,n-type GaAs wafers have been dominantly used for making optoelectronicdevices, for example, photodiodes, light emitting diodes and laserdiodes.

On the contrary, p-type GaAs wafers have a poor demand due to small holemobility. Thus, “GaAs wafers” have indicated n-type GaAs wafers.

We imagine an on-n-GaAs ZnSe photodiode now. If a ZnSe-photodiode weremade by piling n-, i- and p-ZnSe-type material layers on an n-GaAssubstrate, an imaginary PD would have a metallic n-electrode, an n-GaAssingle crystal substrate, an n-ZnSe buffer layer, an n-ZnSe layer, ani-ZnSe layer, a p-ZnSe layer and a metallic p-electrode in series frombottom to top. But, such a ZnSe-PD would not operate with highefficiency.

A drawback is difficulty of making p-type ZnSe. A more serious drawbackin the on-n-GaAs ZnSe PD is that it is impossible to form a p-electrodein ohmic contact with a p-ZnSe layer. The wide bandgap ZnSe prohibits usfrom forming a metallic p-electrode on p-ZnSe. Zinc telluride (ZnTe)which has a narrower bandgap than ZnSe allows us to make a good p-ZnTelayer and make a good p-electrode upon the p-ZnTe layer. For overcomingthe difficulties of the production of p-layers and the formation ofp-electrodes on ZnSe, a superlattice of p-ZnSe/ZnTe having a p-ZnTelayer on top is grown on a p-ZnSe layer. Then, a metallic p-electrode ismade upon the top p-ZnTe layer. The p-electrode makes an ohmic contactwith the undercoating p-ZnTe layer. The metallic p-electrode should bemade into an annular shape or a small dotted shape for allowingincidence light to go via the p-electrode into the p-ZnTe layer.

A virtual pn-junction type or pin-junction type ZnSe-photodiode could bemade upon an n-type GaAs substrate. An n-metallic electrode as a cathodewould be formed upon the n-GaAs substrate. If a reverse bias wereapplied between the metallic p-electrode (anode) and the bottomn-electrode (cathode), a depletion layer (strong electric field area)would be formed at the i-layer or at the pn-junction. If light beamswent into the photodiode and arrived at the depletion layer,photocurrent would be induced. Blue light or violet light could bedetected by the photocurrent.

However, such an on-n-GaAs ZnSe photodiode would have the followingdrawbacks. Incidence light going via an anode (p-electrode) into thephotodiode would pass p-type ZnTe layers which are included in a p-ZnTecontact layer and the (ZnTe/ZnSe) superlattice (MQW: multiquantum well).ZnTe which has a bandgap narrower than ZnSe would absorb the incidencelight which has energy higher than the ZnTe bandgap. The absorption ofthe incidence light by the ZnTe layers is one of serious weak points ofthe on-n-GaAs ZnSe photodiode. A decrease of the light absorption wouldrequire thinning of the ZnTe layers. But, the p-ZnTe contact layer wouldbe indispensable for ohmic contact with the p-electrode and the p-ZnTemultiquantum layers would be also necessary for forming the superlattice(MQW) structure. The absorption caused by the p-ZnTe layers would lowerexternal quantum efficiency. Absorption loss induced by the p-ZnTelayers would be more conspicuous for near-ultraviolet rays.

One purpose of the present invention is to provide an on-GaAs ZnSephotodiode with low absorption loss due to ZnTe. Another purpose of thepresent invention is to provide an on-GaAs ZnSe photodiode with highexternal quantum efficiency.

SUMMARY OF THE INVENTION

ZnSe type photodiodes of the present invention include a metallicp-electrode, a p-GaAs single crystal substrate, a p-(ZnSe/ZnTe)^(m)(piles of pair films, m: integer of the number of pair films)superlattice epitaxially grown on the p-GaAs substrate, ap-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) layer epitaxially grown on thesuperlattice, an i-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) layer epitaxially grownon the p-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) layer, an n- orn⁺-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) layer epitaxially grown on thei-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) layer, a metallic n-electrode formed onthe n-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) layer, and a metallic p-electrodeformed on a bottom of the p-GaAs single crystal substrate.

A feature of the present invention is employment of p-GaAs as asubstrate. Namely, this invention proposes an on-p-GaAs ZnSe photodiode.In the on-p-GaAs ZnSe photodiode, object light rays enter the photodiodevia an n-type layer surface. The-direction of incidence light is reverseto the before-mentioned on-n-GaAs substrate ZnSe photodiode. Thisinvention employs a p-GaAs wafer as a starting substrate. The presentinvention produces a p-type superlattice, p-type layers, an i-typelayer, n-type layers in this turn on the p-GaAs substrate. Theinterposition of the p-superlattice between the p-GaAs substrate and thep-type layers characterizes this invention. An inlet surface is not thep-substrate surface but the top n-layer surface. Incidence light goesvia the top n-layers into a pn-junction or pin-junction. Thesuperlattice does not exist in a path from an inlet to an active layer.The superlattice lies under the active layer. The incidence light doesnot pass the superlattice which is formed under the pn- or pin-junction.The present invention is free from the absorption of the object light bythe superlattice including ZnTe.

A metallic n-electrode should be an annular one or a small doted oneformed on the top n-layer for allowing incidence light to enter thephotodiode via the top n-layer. An extra space on the top layer may beeither uncoated or coated with an antireflection film or a protectionfilm.

The photodiode of the present invention has a three-layerednip-structure. A superlattice is formed on a p-GaAs substrate. Annip-layered structure is formed upon the superlattice. All the three n-,i-, p-layers can take ZnSe, ZnS_(y)Se_(1-y) (0≦y≦0.8) orZn_(1-x)Mg_(x)S_(y)Se_(1-y) (0≦x, y≦0.8). The mixture ratios x, y arenot necessarily common values to all the three layers.

Zn_(1-x)Mg_(x)S_(y)Se_(1-y) is reduced to ZnS_(y)Se_(1-y) for x=0. Whenx=0 and y=0, Zn_(1-x)Mg_(y)S_(y)Se_(1-y) is reduced simply to ZnSe.Zn_(1-x)Mg_(x)S_(y)Se_(1-y) is a collective representation includingZnS_(y)Se_(1-y), Zn_(1-x)Mg_(x)Se and ZnSe.

An on-p-GaAs pin ZnSe photodiode of the present invention has a layeredstructure from bottom to top as follows.

-   -   1. metallic p-electrode (bottom electrode) Au—Pd—Pt, Au—Ti,        Au—Pt, or Au    -   2. p-type substrate p-GaAs substrate    -   3. p-type superlattice p-(ZnSe/ZnTe)^(m) (MQW: multiquantum        well)        -   (m; integer of the number of sets of layers)    -   4. p-type layer p-ZnSe, p-ZnSSe, p-ZnMgSSe    -   5. i-type layer i-ZnSe, i-ZnSSe, i-ZnMgSSe    -   6. n-type contact layer n⁺-ZnSe, n⁺-ZnSSe, n⁺-ZnMgSSe    -   7. antireflection film Al₂O₃, SiO₂, TiO₂, La₂O₃, MgF₂ or set of        them    -   8. top n-electrode Au—In, In, In—Au—Ge

The i-layer is a non-doped layer which has the least carrierconcentration.

The carrier concentration of the i-layer is equal to or lower than 10¹⁶cm⁻³. What has the highest carrier concentration is the n-type contactlayer. Ohmic contact with the metallic n-electrode requires high carrierconcentration for the n-contact layer. A suitable carrier density of then-contact layer ranges from n=10¹⁸ cm⁻³ to 10²⁰ cm⁻³. The p-layers and abuffer p-layer have a carrier concentration of p=10¹⁶ cm⁻³ to 10¹⁸ cm⁻³.The p-superlattice and the p-GaAs substrate have a carrier concentrationof p=10¹⁷ cm⁻³ to p=5×10¹⁹ cm⁻³. The present invention employs nitrogen(N) as a p-dopant and chlorine (Cl) as an n-dopant.

Bandgaps of the n-, i- and p-layers are described. A set of n-, i- andp-layers having the same bandgap is allowable. Namely, the set of n-, i-and p-layers composed of the same components is available. If the n-,i-, p-layers have a common set of components and a common bandgap, theupper n-layer absorbs a part of incident light, which decreases quantumefficiency and sensitivity. The absorption can be reduced by thinningthe top n-layers. But, thinning of the n-layers cannot decrease theabsorption loss to zero. For avoiding the n-layer absorption, it iseffective to increase the bandgap of the top n-layer (an n-windowlayer). Use of quaternary components enables us to enlarge the bandgapof the n-layer by changing a mixture rate. An increase of a Mg rate oran S rate to Se raises the bandgap of the n-layer.

Two cases are available for selecting bandgaps Ep, Ei and En of the p-,i- and n-layers.

-   -   (1) common bandgap case Ep=Ei=En

All the three layers have a common bandgap. In the common bandgap case,all the n-, i-, p-layers can be made of ZnSe. Otherwise, all the threelayers can be made of ZnSSe of the same rate. Or the three layers can beformed of ZnMgSSe.

-   -   (2) different bandgap case En>Ei=Ep

The n-layer has a bandgap larger than the bandgap of the i-layer and thep-layer. An allowable set has an n-layer made of ZnSSe and a p-layer andan i-layer made of ZnSe. Otherwise, an i-layer and a p-layer are ZnSSeand an n-layer is ZnMgSSe.

For obtaining enough sensitivity up to near-ultraviolet (about 300 nm),the sensing layer (i-ZnMgSSe layer) should have a Mg rate of more than0.1 and a S rate of more than 0.1 (0.1≦x, 0.1≦y; inZn_(1-x)Mg_(x)S_(y)Se_(1-y)). For example, ani-Zn_(0.9)Mg_(0.1)S_(0.1)Se_(0.9) layer can build a ZnSe type photodiodewith high sensitivity for 300 nm.

As mentioned before, the on-n-GaAs ZnSe PD, which would be made bypiling an n-layer, an i-layer, a p-layer, p-(ZnTe/ZnSe)^(m) superlatticeand a —ZnTe contact layer on an n-GaAs substrate, would be annoyed bypoor quantum efficiency, because the upper p-ZnSe layers in thesuperlattice and in the contact layer would absorb incidence light. Thetop superlattice would be indispensable for alleviating the energy gapbetween the top p-ZnTe contact layer and the p-ZnSe layer in theon-n-GaAs ZnSe PD. Without the p-(ZnTe/ZnSe)^(m) superlattice,photocurrent could not flow by the energy gap.

Instead of the n-GaAs substrate, the present invention starts from ap-GaAs substrate and grows p-, i-, n-ZnMgSSe layers in this turn uponthe p-GaAs substrate. Use of an n-ZnSe contact layer has a merit ofestablishing an ohmic contact n-electrode with a pertinent metal, forexample, indium as an n-electrode. There is no p-(ZnTe/ZnSe)^(m)superlattice on the top of the nip/p-GaAs PD. Since incidence lightcoming via a top n-layer does not pass the p-(ZnTe/ZnSe)^(m)superlattice containing ZnTe, no absorption loss occurs in the presentinvention.

However, the present invention does not choose a simple on-p-GaAssubstrate structure having a bottom p-GaAs substrate and p-, n-, or p-,i-, n-ZnMgSSe layers on the p-GaAs substrate. The inventors of thepresent invention actually had made such a primitive on-p-GaAs PD andhad examined the prototype on-p-GaAs PD.

The inventors of the present invention noticed that the simple on-GaAsZnSe photodiode having p-, n-, or p-, i-, n-ZnMgSSe layers on a p-GaAssubstrate was inoperative.

The prototype ZnSe photodiode the inventors had produced and examinedwas a ZnSe photodiode having a p-GaAs substrate, p-, n-, or p-, i-,n-ZnSe layers grown directly in series on the p-GaAs substrate, ann-electrode on the n-ZnSe and a p-electrode formed on a bottom of thep-GaAs substrate. A serious problem had occurred in a forwardvoltage/current performance. When a forward voltage of 5 V to 10 V wasapplied to the prototype PD, little current flows in a forwarddirection. The forward current means a current which flows from ap-electrode (anode) via p-GaAs and n-ZnSe to an n-electrode (cathode).In the prototype photodiode, unless a forward voltage bigger than 5V to10V was applied, no forward current flowed. Namely, rectifying propertywhich is one of the significant properties of diodes was insufficient.

The inventors had sought the reason of inducing the poor forwardcurrent. The inventors had noticed that a large barrier was made at aninterface between the p-GaAs and the p-ZnSe layer and the ZnSe/GaAsbarrier stopped a flow of holes across the interface. GaAs has a bandgapdifferent from ZnSe. The difference of the bandgap makes a large barrierat the GaAs/ZnSe interface.

The inventors have hit upon an idea of interposing a superlattice whichpiles ZnSe/ZnTe layers in turn at the ZnSe/GaAs interface forannihilating the ZnSe/GaAs barrier. The ZnSe/ZnTe superlattice is anassembly of thin ZnSe films and ZnTe thin films reciprocally piled inturn. Here, the superlattice is denoted by (ZnSe/ZnTe)^(m), where m isan integer for denoting the number of pairs of ZnSe and ZnTe films.Thicknesses of the thin films change stepwise. A suitable example of thesuperlattice will be described later. The (ZnSe/ZnTe)^(m) superlatticeenables holes to run smoothly through the superlattice as tunnelcurrent. The inventors found that the (ZnSe/ZnTe)^(m) superlattice cansolve the difficulty of the ZnSe/GaAs barrier. A gist of the presentinvention is the interposition of the superlattice between the p-GaAssubstrate and the p-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) layer.

Incidence light enters via an n-side aperture made on a part of then-layer. (Another part of the n-layer is covered with the n-electrode.)Optionally an antireflection film or a transparent protection film isformed on the n-side aperture. Transparent, rigid films, for example,Al₂O₃, SiO₂, TiO₂, La₂O₃ or MgF₂ films, are suitable for protection orantireflection films. An antireflection film is made of a pile ofdifferent dielectric thin films with pertinent refractive indices andthicknesses. A simple antireflection film can be made by piling a singledielectric film having a ¼n thickness on the n-side aperture. Here, “n”is a refractive index of the film.

The present invention has a fundamental structure ofPIN/superlattice/p-GaAs junction. The fundamental structure can becommonly applied to an avalanche photodiode (APD), which has thefollowing structure.

-   -   1. p-side metallic electrode (bottom) Au—Pd—Pt, Au—Pt, Au—Ti, Au    -   2. p-type substrate p-GaAs    -   3. p-superlattice (MQW: multiquantum well) p-(ZnSe/ZnTe)^(m) (m:        paring layer number)    -   4. p-layer p-ZnSe, p-ZnSSe, p-ZnMgSSe    -   5. n⁻-layer n⁻-ZnSe, n⁻-ZnSSe, n⁻-ZnMgSSe    -   6. n⁺-contact layer n⁺-ZnSe, n⁺-ZnSSe, n⁺-ZnMgSSe    -   7. antireflection film (optional) Al₂O₃, SiO₂, TiO₂, La₂O₃, MgF₂        or set of them    -   8. n-side metallic electrode(top) Au—In, In, In—Au—Ge

The avalanche photodiode is illustrated in FIG. 18. In the avalanchediode, a strong reverse bias is applied between an anode (p-electrode)and a cathode (n-electrode) for inducing avalanche amplification. Thestrong bias makes a strong electric field at the pn-junction. The strongelectric field makes a wide depletion layer in the vicinity of thepn-junction. In this case of n⁻-avalanche layer, the depletion layer ismainly produced in the lower doped n⁻-layer. An increase of the reversebias raises the breadth of the depletion layer. Incidence lightgenerates pairs of electrons and holes in the n⁻-depletion layer. Theholes which are minority carriers are accelerated by the reverse biasvoltage. The rapid holes collide with lattice atoms and extract holesand electrons. New carriers are accelerated and make further newcarriers. An avalanche amplification takes place in the n⁻-layer.Optionally, an additional thin i-layer can be interposed between4.p-layer and 5.n⁻-layer. The i-layer has an effect of lowering andstabilizing dark current by improving performance of crystallographicproperty of the pn-interface. Otherwise, it is also advantageous toinsert a p-ZnSe buffer layer between 3.superlattice and 4.p-layer. Theinserted buffer layer should be made at a lower temperature than thetemperature at which other layers are epitaxially grown. Thelow-temperature grown buffer layer enhances a crystallographical qualityof Zn_(1-x)Mg_(x)S_(y)Se_(1-y) layers grown upon the buffer layer.

[Bandgap and Lattice Constant of Zn_(1-x)Mg_(x)S_(y)Se_(1-y) QuaternaryMixture Crystals]

This invention employs ZnSe type crystals as n-,i-,p-layers (“nip-”abbr.) which should be grown on a p-GaAs substrate crystal. The nip-ZnSetype crystals mean not only ZnSe itself but also ZnSe-like mixturecrystals having wider bandgaps than ZnSe. Nip-layers should satisfylattice fitting with GaAs. Namely, the lattice constants of thenip-layers should be nearly equal to the lattice constant of GaAs. FIG.1 is a graph showing the relation between the lattice constants andbandgaps of II-VI (2-6) group compounds. The abscissa is latticeconstants (nm). The left ordinate is bandgap energy (eV). The rightordinate denotes absorption edge wavelengths (nm). Round dots meancompounds of II-VI group. A rhombus formed by ZnSe, ZnS, MgS and MgSe isdepicted at a central portion. ZnSe has a bandgap of 2.68 eV (460 nm)and a lattice constant of 0.5668 nm. The ZnSe lattice constant is nearlyequal to GaAs. A transition (Zn_(1-x)Mg_(x)Se) from ZnSe to MgSeincreases both lattice constants and bandgaps. MgS has a 270 nmabsorption edge wavelength which is the shortest in allZn_(1-x)Mg_(x)S_(y)Se_(1-y) mixtures.

Another transition (ZnS_(y)Se_(1-y)) from ZnSe to ZnS decreases latticeconstants but increases bandgap energies. The ZnSe-ZnS transition is notlinear. ZnS has an absorption edge wavelength of about 340 nm. Thelattice constant of ZnS is smaller than ZnSe. MgS has a bandgap (4.5 eV)larger than ZnS and MgSe. The lattice constant of MgS is about 0.56 nmwhich is akin to GaAs. All materials included within the rhombuscorrespond to quaternary compounds Zn_(1-x)Mg_(x)S_(y)Se_(1-y) whichinclude two parameters x and y showing mixture rates. Sometimes,Zn_(1-x)Mg_(x)S_(y)Se_(1-y) is abbreviated to ZnMgSSe by omitting therepresentation of mixture rates x and y.

GaAs is designated by a point [A] just below the rhombus in FIG. 1. GaAshas a 0.56 nm lattice constant and a bandgap of 1.42 eV. An extension ofa line connecting ZnSe and MgS coincides with the GaAs lattice constantline of 0.56 nm. The quaternary compounds Zn_(1-x)Mg_(x)S_(y)Se_(1-y)have a potential for changing bandgap energy from 2.5 eV (ZnSe) to 4.5eV (MgS) under the restriction of the lattice matching with GaAs. Thisis an advantageous feature of Zn_(1-x)Mg_(x)S_(y)Se_(1-y) type crystals.ZnSe type photodiodes of the present invention can choose ZnSe, ZnSSe,ZnMgSe or ZnMgSSe as nip-layers. The range of suitable mixtures is0≦x≦0.8 and 0≦y≦0.8.

This invention proposes an on-p-GaAs ZnSe type pin-photodiode which isproduced by piling a p-ZnSe/ZnTe-SL (MQW), a p-ZnMgSSe layer, ani-ZnMgSSe layer and an n-ZnMgSSe layer in series on a GaAs substrate andan on-p-GaAs ZnSe type avalanche photodiode which is produced by pilinga p-ZnSe/ZnTe-MQW, a p-ZnMgSSe layer, an n⁻-ZnMgSSe layer and ann⁺-ZnMgSSe layer in series on a GaAs substrate. The present inventionovercomes the weak point of Si-PDs (poor sensitivity for blue toultraviolet) and the drawback of on-sapphire GaN-PDs (large dark currentinduced by big lattice misfitting). Furthermore, this invention solvesthe difficulty of an on-n-GaAs ZnSe PD (low quantum efficiency caused byabsorption of ZnTe layers). The pin-PD and APD of the present inventionhave advantages of good quantum efficiency, high sensitivity forblue/ultraviolet, high reliability, low dark current and long lifetime.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a bandgap/lattice diagram for showing relations betweenlattice constants and bandgap energy of ZnMgSSe-type quaternarycomponent mixture crystals.

FIG. 2 is a sectional view of an on p-GaAs/ZnSe-photodiode of Embodiment1 of the present invention. Embodiment 1 interposes a p-ZnTe/ZnSe-MQWbetween a p-type GaAs substrate and a p-type ZnSe buffer layer.

FIG. 3 is a sectional view of an on p-GaAs/ZnSe-photodiode of acomparison example 1 which lacks a p-ZnTe/ZnSe-MQW between a p-type GaAssubstrate and a p-type ZnSe buffer layer and brings a p-ZnSe bufferlayer in direct contact with a p-type GaAs substrate.

FIG. 4 is a graph showing forward currents in a range of 10⁻⁷ A to 10⁻⁶A as a function of forward voltages of ZnSe-photodiodes of Embodiment 1and Comparison Example 1. The abscissa is a forward voltage. Theordinate is a forward current.

FIG. 5 is a graph showing forward currents in a logarithmic scale in arange of 10¹² A to 10⁻³ A as a function of forward voltages ofZnSe-photodiodes of Embodiment 1 and Comparison Example 1. The abscissais a forward voltage. The ordinate is a forward current.

FIG. 6 is a graph showing the dependence of external quantum efficiencyupon wavelengths for parameters of reverse biases of the ZnSe-photodiodeof Embodiment 1. The abscissa denotes a wavelength (nm), and theordinate denotes external quantum efficiency.

FIG. 7 is a graph showing changes of dark currents of a ZnSe-PD, aGaN-PD and a Si-PD as a function of reverse biases at room temperature.An abscissa is a reverse bias (V). An ordinate is dark current density(A/cm²).

FIG. 8 is a sectional view of an molecular beam epitaxy (MBE) apparatuswhich the inventors employ for growing p-ZnSe/ZnTe superlattices (MQW),p-type ZnSe buffer layers, p-type ZnMgSSe layers, i-type ZnMgSSe layersand n-type ZnMgSSe layers on p-type GaAs single crystal substrates.

FIG. 9 is a sectional view of a ZnSe/ZnTe superlattice (MQW) structureof an on-p-GaAs substrate ZnSe photodiode of Embodiment 1 of the presentinvention.

FIG. 10 is a band scheme of a direct junction of p-GaAs and p-ZnSe forshowing discontinuity occurring on a valence band preventing holes fromflowing from p-GaAs to p-ZnSe.

FIG. 11 is a band structure of a junction of p-GaAs and p-(ZnSe/ZnTe)superlattice (MQW; multiquantum well) for inducing tunnel conduction ofholes from p-GaAs to p-ZnSe and avoiding a valence band discontinuityfrom suppressing a hole flow from p-GaAs to p-ZnSe.

FIG. 12 is a graph showing a result of experiments of reflection loss ofZnSSe-PDs of Embodiment 1 (without antireflection film) and Embodiment 2(with antireflection film) as a function of wavelengths of light. Theordinate is a reflection loss (%). The abscissa is a wavelength (nm) ofincident light. Embodiment 2 with an antireflection film enjoys lowerreflection loss than Embodiment 1 without an antireflection film.

FIG. 13 is a graph denoting external quantum efficiency of the ZnSSe-PDsof Embodiment 1 without an antireflection film (AR film) and Embodiment2 with an antireflection film as a function of wavelengths of light. Theabscissa is a wavelength (nm). The ordinate is external quantumefficiency (%). Embodiment 2 having an antireflection film shows quantumefficiency higher than Embodiment 1 without AR film.

FIG. 14 is a graph showing photocurrents of the ZnSSe-PD of Embodiment 2as a function of incidence light power. The ordinate is photocurrents(μA). The abscissa is incidence light power (μW).

FIG. 15 is a graph of quantum efficiencies n_(ex) of the ZnSSe-PD ofEmbodiment 2 measured at 300K by changing incidence light power. Theordinate is quantum efficiency η_(ex) (%). The abscissa is incidencelight power (μW). 10 μW to 10³ μW of incidence light power lift thequantum efficiencies over eighty percent (η_(ex)>80%).

FIG. 16 is a graph of quantum efficiencies η_(ex) of the ZnSSe-PD ofEmbodiment 2 measured by irradiating by a He—Cd laser beam at pointsalong a diameter of a reception aperture of the PD. The ordinate isquantum efficiency η_(ex) (%). The abscissa is distances of the points(mm) irradiated by the He—Cd laser beam. The quantum efficiencies areenough high (about 78%) at a central region within a 0.6 mm radiusaround the center which is 1 mm distanced from the origin.

FIG. 17 is a graph of reflection loss of the ZnSSe-PD of Embodiment 2measured by irradiating by a He—Cd laser beam at points along a diameterof the reception aperture of the PD. The ordinate is reflection loss(%). The abscissa is distances of the points (mm) irradiated by theHe—Cd laser beam. The reflection loss is lower than 5% at a centralregion within a 0.6 mm radius around the center.

FIG. 18 is a schematic sectional view for showing a common layeredstructure of a ZnSSe type avalanche photodiode of the present invention.

FIG. 19 is a sectional view for showing a common layered structure of aZnSSe avalanche photodiode of Embodiment 3.

FIG. 20 is a graph for showing avalanche breakdown property of the ZnSSeavalanche photodiode of Embodiment 3 measured by varying surroundingtemperatures (K).

FIG. 21 is a graph of electric field dependence of multiplication rates(M) of the ZnSSe avalanche photodiode of Embodiment 3.

FIG. 22 is a graph showing wavelength dependence of the external quantumefficiency of the ZnSSe-APD of Embodiment 4 of the present invention.The abscissa is a wavelength of incidence light. The ordinate isexternal quantum efficiency (%) of the APD. Parameters 0V, −30V, −32V,−34V and −35V affixed the curves denote reverse biases.

FIG. 23 is a sectional view of a ZnSe/ZnTe superlattice (MQW) structureof an on-p-GaAs substrate ZnSSe photodiode of Embodiment 2 of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention makes a ZnSe-type photodiode upon a(ZnTe/ZnSe)^(m) superlattice (MQW) formed upon a p-GaAs substrate. The(ZnTe/ZnSe)^(m) superlattice excludes occurrence of a barrier betweenGaAs and ZnSe which impedes holes from flowing from GaAs to ZnSe in aforward direction. Without the (ZnTe/ZnSe)^(m) superlattice, a barrierstops holes. No practical on-p-GaAs photodiode can be produced due tothe (ZnSe/GaAs) barrier. A comparison example without superlattice isproduced as Comparison example 1 for clarifying effects of asuperlattice included in a photodiode (Embodiment 1) of the presentinvention.

Structure of a ZnSe-Type Photodiode of Embodiment 1 (FIG. 2)

FIG. 2 shows a layered structure of a Zn_(1-x)Mg_(x)S_(y)Se_(1-y)photodiode of Embodiment 1 from top to bottom. n-electrode indium (In)dotted shape φ = 0.8 mm n⁺-ZnSSe n = 2 × 10¹⁹ cm⁻³ thickness 44 nmi-ZnSSe n < 10¹⁶ cm⁻³ thickness 900 nm p-ZnSSe p = 3 × 10¹⁷ cm⁻³thickness 450 nm p-ZnSe (buffer) p = 5 × 10¹⁷ cm⁻³ thickness 30 nmp-(ZnSe/ZnTe)^(m) MQW total thickness 12 nm p-GaAs substrate p = 2 ×10¹⁹ cm⁻³ p-electrode indium (In) (allover)

The mixture ratio is x=0 and y=0.055 in Embodiment 1. All n-, i- andp-layers are triplet compounds ZnS_(0.005)Se_(0.945). The 900 nm thicki-ZnSSe layer is a non-doped layer. The carrier concentration is lessthan 10¹⁵ cm⁻³ (n<10¹⁶ cm⁻³). Thus, the i-layer is a depletion layer.The thickest i-ZnSSe layer acts as an active layer which absorbs bluelight and ultraviolet rays and produces pairs of holes and electrons.Application of a reverse bias widens the depletion layer beyond thei-layer slightly inward to the p-layer.

Structure of a ZnSe-Type Photodiode of Comparison Example 1 (FIG. 3)

Comparison Example 1 (not prior art) the inventors made has a layeredstructure from top to bottom as shown in FIG. 3. n-electrode indium (In)dotted shape φ = 1.0 mm n⁺-ZnSSe n = 2 × 10¹⁹ cm⁻³ thickness 20 nmi-ZnSSe n < 10¹⁶ cm⁻³ thickness 1000 nm p-ZnSSe p = 3 × 10¹⁷ cm⁻³thickness 500 nm p-ZnSe (buffer) p = 5 × 10¹⁷ cm⁻³ thickness 50 nmp-GaAs substrate p = 2 × 10¹⁹ cm⁻³ p-electrode indium (In) (allover)

Comparison Example 1 is not prior art. Thicknesses are slightlydifferent from Embodiment 1. A large difference is omission of the(ZnSe/ZnTe) superlattice. The mixture rate of Comparison Example 1 isy=0.055 which is the same as Embodiment 1.

Forward Voltage/Current Performance of Embodiment 1 and ComparisonExample 1 (FIG. 4 and FIG. 5)

FIG. 4 and FIG. 5 denote forward voltage/current properties of theZnSe-PDs of Embodiment 1 and Comparison Example 1. FIG. 4 shows apartial range between 10⁻⁷ A and 10⁻⁶ A of the forward current graph.The abscissa is forward voltages (V). The ordinate is forward currents(×10⁻⁷ A). A left branch shows a measured result of Embodiment 1. Aright branch shows a measured result of Comparison Example 1. Embodiment1 takes forward current between 10⁻⁷ A and 10⁻⁶ A for forward voltagefrom 1.6V to 1.8V. On the contrary, comparison Example 1 gives the samerange (between 10⁻⁷ A and 10⁻⁶ A) of forward current for forward voltagefrom 6 V to 7.5V which is far larger than Embodiment 1.

FIG. 5 shows a wide range of forward currents from 10⁻¹² A to 10⁻³ A.The abscissa is forward voltage (V). The ordinate is forward current ina logarithmic scale.

A left branch denotes the property of Embodiment 1 in FIG. 5. 0V forwardvoltage induces 10⁻¹² A forward current. 1.2V forward voltage-gives4×10⁻¹² A forward current. The forward current rises rapidly then. 2Vforward voltage increases forward current up to 10⁻⁴ A. Embodiment 1exhibits a good forward current performance.

A right branch shows Comparison Example 1 in FIG. 5. No forward currentflows for forward voltage from 0V to 4V. Comparison Example 1 has a highbarrier for forward current. 5V forward voltage yields 4×10⁻¹² A offorward current. 6V forward voltage induces 4×10⁻⁹ A forward current. 8Vforward voltage makes 4×10⁻⁶ A forward current. A rising voltage ishigh. A rising current curve is slow. Comparison Example 1 lacks gooddiode property. The bad diode property (poor rectifying property) wouldoriginate from the barrier between GaAs and ZnSe. Embodiment 1 raisesforward current rapidly at a low voltage. Embodiment 1 farther surpassesComparison Example1 in the diode property. The large and rapid risingforward current results from the ZnSe/ZnTe multiquantum well which isinterposed between GaAs and ZnSe. The good diode property of Embodiment1 is an excellent advantage which enables us to make practicalphotodiodes for ultraviolet and blue light.

External Quantum Efficiency of ZnSSe Type Photodiodes of Embodiment 1(FIG. 6)

FIG. 6 shows wavelength dependence of external quantum efficiency ofZnSSe photodiodes of Embodiment 1. Affixed parameters show reversebiases (0V, −5V, −10V, −15V and −20V). The abscissa is wavelengths (nm)of incidence light in a range between 300 nm and 500 nm. The ordinate isexternal quantum efficiencies (%). Embodiment 1 has no antireflectionfilm on the top aperture. An addition of an antireflection film raisesthe external quantum efficiencies by 10% to 15%. Even without bias,external quantum efficiency is 38% at a 450 nm wavelength. A −5V reversebias enhances external quantum efficiency up to 53% at 450 nm. Anincrease of reverse bias raises external quantum efficiency. A −20Vreverse bias allows external efficiency to take a maximum 67.6% at 449nm light. External quantum efficiency for 400 nm light by a −20V reversebias is 56.9%. High external quantum efficiency is equivalent to highsensitivity for the light in the wavelength range. High external quantumefficiencies in the range between 300 nm and 450 nm show excellency ofthe ZnSe-type PD of Embodiment 1 as a violet/blue photodiode. The highexternal quantum efficiency originates from non-existence of a ZnTelayer and an MQW having ZnTe in an upper part above the sensing i-layer.Incidence light arrives at the sensing i-layer without passingabsorptive ZnTe layers. No absorption by ZnTe layers occurs inEmbodiment 1. Since object light enters via n-layers which need notinterpolated ZnTe, incidence light can reach the sensing layer withoutabsorption by ZnTe. The quantum efficiency is raised by excludingabsorption by ZnTe layers in the present invention.

Comparison of Dark Currents of Embodiment 1 with Other Photodiodes (FIG.7)

Low dark current is another advantage of the present invention. FIG. 7shows dark currents of a GaN-PD, a Si-PD a ZnSe-PD (the presentinvention) as a function of reverse biases. The GaN-PD which hassensitivity for ultraviolet has a large dark current. A reverse bias of10V induces a large dark current of 2×10⁻⁶/cm² in the GaN-PD. When 13Vof reverse bias is applied, 10⁻⁵ A/cm² dark current flows. Since a largeGaN substrate single crystal is not obtainable, a sapphire singlecrystal is used for a substrate of the GaN-PDs. Lattice-misfittinginduces a high dislocation density and causes a large dark current. TheGaN-PDs cannot be used as a high sensitivity photodetector due to thebig dark current. A silicon photodiode (Si-PD) has nearly constant darkcurrent ranging from 5×10⁻¹⁰ A/cm² to 2×10⁻⁹ A/cm² for reverse biasesfrom 1V to 20V. The Si-PD has poor sensitivity for violet andultraviolet because of the narrow bandgap. The Si-PD is ineffective fordetecting violet or ultraviolet rays despite small dark current.

The ZnSe-type photodiodes of Embodiment 1 has a good dark currentproperty. For 5V of reverse bias, dark current is very small (7×10⁻¹¹A/cm²). 10V of reverse bias causes 2×10¹⁰ A/cm². At 20V reverse bias,dark current of the ZnSe-PD is smaller than the Si-PD. TheZnSe-photodiode of the present invention is excellent in the externalquantum efficiency and the dark current.

Structure of ZnSSe Type Photodiode of Embodiment 2 (Antireflection Film)

n-electrode indium (In) dot-shape φ=1.0 mm

antireflection film (AR) TiO₂/SiO₂ layers thickness 60 nm

n⁺-ZnSSe n=2×10¹⁹ cm⁻³ thickness 44 nm

i-ZnSSe n<10¹⁵ cm⁻³ thickness 9000 nm

p-ZnSSe p=3×10¹⁷ cm⁻³ thickness 450 nm

p-ZnSe p=5×10¹⁷ cm⁻³ buffer thickness 30 nm

p-ZnSe/ZnTe MQW thickness 13.5 nm

p-GaAs substrate p=2×10¹⁹ cm⁻³

p-metallic electrode indium (In)

Embodiment 2 forms a 60 nm thick TiO₂/SiO₂ antireflection film on a topaperture in addition to the structure of Embodiment 1. Embodiment 2 issimilar to Embodiment 1 (FIG. 1) except the antireflection film aroundthe In n-electrode. Thus, Embodiment 2 is not shown in a figure. Thesuperlattice has five pairs of p-ZnTe/ZnSe layers. Namely, thesuperlattice is denoted by p-(ZnTe/ZnSe)⁵. All five p-ZnSe layers have a2 nm thickness in common. But, five p-ZnTe layers have thicknesses of0.2 nm, 0.4 nm, 0.6 nm, 0.8 nm and 1.0 nm from top to bottomrespectively as illustrated in FIG. 23.

Properties of Embodiment 1 (without antireflection AR film) andEmbodiment 2 (with antireflection AR film) are compared. FIG. 12 shows aresult of measurement of reflection loss of two kinds of nip-ZnSSephotodiodes. The abscissa is wavelengths (nm) of incidence light from300 nm to 500 nm. The ordinate is reflection loss (%). Since futuristicDVD devices based upon GaN-LDs would utilize readout light of awavelength of 400 nm, reflection loss is measured in a 200 nm wide rangeat 400 nm±100 nm. A solid line denotes Embodiment 2 (with AR). A dottedline shows Embodiment 1 (without AR). Non AR Embodiment 1 showsreflection loss of about 23% to 25% for 300 nm to 450 nm. The reflectionloss vibrates from 460 nm to 500 nm. The vibration is induced byinterference.

AR coated Embodiment 2 shows a maximum reflection loss of 28% at 300 nm.Reflection loss linearly decreases as the wavelength increases. Thedecrease is an advantage of forming the antireflection film on the topaperture. The reflection loss is smaller than AR-less Embodiment 1 in awide range from 340 nm to 500 nm. At 400 nm of the assumed DVD readoutwavelength, the reflection loss is 11%. The AR film reduces reflectionloss at 400 nm by about 10%. At 460 nm, the reflection loss takes aminimum value of 2%. Reduction of the reflection loss exhibits aconspicuous advantage of the antireflection film.

FIG. 13 is a result of measurement of external quantum efficiencies ofthe nip-ZnSSe photodiodes (PDs) of Embodiments 1 and 2. A solid linedenotes Embodiment 2 with an antireflection film. A dotted linedesignates Embodiment 1 without an antireflection film. External quantumefficiency falls linearly in proportion to a decrease of wavelengthsbetween 450 nm and 300 nm. The decline is caused by an increase ofabsorption by a window layer (top ZnSSe). Longer wavelengths are lesssubject to absorption. Embodiment 2 with an antireflection film obtains83% external quantum efficiency at 450 nm. This is an excellentproperty. Embodiment 1 without AR film shows 66% of external quantumefficiency at 450 nm. The difference derives from difference of additionof an AR film which decreases absorption loss.

FIG. 14 shows a result of measurement of photocurrents of thenip-ZnSSe-PDs of Embodiment 2 (with AR coating) as a function of thepower (μW) of incidence light. The nip-PD is reverse biased byV_(R=)=−20 V. The photocurrent is measured at 300K. The light source isa He—Cd laser emitting a beam of a 0.2 mm diameter of a 442 nmwavelength. The photocurrent linearly increases in proportion to theincidence light power in Embodiment 2.

FIG. 15 shows a graph of external quantum efficiency (η_(ex)) ofEmbodiment 2 (AR coating) by changing input power (μW). The quantumefficiency is measured by applying a −20V reverse bias at 300K. Thelight source is a He—Cd laser which emits a 0.2 mm φ beam of awavelength of 442 nm. The quantum efficiency exceeds 80% in a wide rangeof 10 μW to 10³ μW of input light power. The high efficiency showsexcellence of Embodiment 2.

FIG. 16 is a graph of spatial variations of external quantum efficiencyalong a diameter of the ZnSSe-PD of AR-coated Embodiment 2 which ismeasured by irradiating spots along the diameter on the top aperture bya converged beam of a He—Cd laser. The irradiating wavelength is 442 nm.The reverse bias is −20V. The diameter of the He—Cd beam is 0.2 mmφ. Apoint of 1 mm is a center of the aperture of the PD. The externalefficiency η_(ex) is 78% within a circle of a 0.6 mm radius around thecenter.

FIG. 17 is a graph for showing spatial variations of reflection lossalong a diameter on the top aperture of the ZnSSe-PD of AR-coatedEmbodiment 2.

Reflection loss at the center (1 mm) is 5%. The reflection loss is 5%within a circle of a 0.6 mm radius around the center. This resultexhibits a superb function of the antireflection film. At points of 0 mmand 1.8 mm, reflection loss rises, which is caused by the reflection bya metal flange of the photodiode package.

Method of Producing Photodiodes of Embodiment 1 (Common for AllEmbodiments)

1. Epitaxial Growth (Production of Layers, Molecular Beam Epitaxy (MBE))

P-type GaAs substrate wafers are prepared. A p-ZnSe/ZnTe MQW, a p-ZnSebuffer layer, a p-ZnSe layer, an i-ZnSSe layer and an n-ZnSSe layer arein turn on the p-GaAs substrate wafer by an MBE method. A molecular beamepitaxy (MBE) apparatus is described by referring to FIG. 8. The MBEapparatus has a MBE chamber 92 which can be evacuated up to ultrahighvacuum. Liquid nitrogen shrouds 93 are equipped within the MBEapparatus. A vacuum apparatus (not shown in the figure) makes vacuum inthe chamber up to 10⁻⁸ Pa (about 10⁻¹⁰ Torr) by operation of two kindsof vacuum pumps.

The MBE chamber has a substrate holder 94 at a center. A p-GaAssubstrate wafer 95 is fitted on the bottom of the substrate holder 94. Aplurality of molecular beam cells (K-cells) 96, 97 and 98 are installedat spots on a bottom of an imaginary cone having a top point at thesubstrate holder 94. The K-cells have functions of heating materials,making a molecular beam and shooting the molecular beams of matrixmaterials or dopant materials toward the GaAs substrate 95. FIG. 8denotes a ZnCl₂-cell 96, a Se-cell 97, a Zn-cell 98 and so on. Thesecells are necessary to produce ZnSe and ZnSSe layers.

A Cd-cell, a Mg-cell, a S-cell and a Te-cell are provided below thesubstrate holder 94 besides the ZnCl-cell, Se-cell, Zn-cell. These cellsare utilized for producing ZnSSe or ZnMgSSe layers. The ZnCl₂ cell 96 isprovided for doping ZnSe, ZnSSe or ZnMgSSe layers with chlorine (Cl) asan n-dopant. Cl becomes an n-type dopant by replacing group 6 elements(S or Se).

Nitrogen (N) is a p-dopant for ZnSe, ZnSSe, and ZnMgSSe. A radical cell99 is employed for nitrogen doping. The radical cell has a coil foractivating nitrogen molecules (N₂). Nitrogen molecules (N₂) are excitedand activated into nitrogen radicals N* by supplying high frequencyelectric power to the coil. Nitrogen radicals are active. When ZnSe typecompounds are doped with nitrogen radicals, the nitrogen radicals becomep-dopants by replacing group 6 elements. Nitrogen had not been able tobe a p-dopant until nitrogen radical cells were invented. The radicalcell enables nitrogen to be a p-dopant in ZnSe type compounds. When ann-layer is grown, the object layer should be doped with chlorine by theZnCl₂ cell 96. When a p-layer is grown, the layer should be doped withnitrogen by the N-radical cell 99.

The growth temperature ranges from 275° C. to 325° C. The ratio of(group VI/group II) is 1 to 5. The growing speed is 0.4 μm/H to 1 μm/H.Embodiment 1 grows a 12 nm thick multiquantum well (MQW), a 30 nm thickp-ZnSe buffer layer, a 450 nm thick p-ZnSSe layer, a 900 nm thicki-ZnSSe layer and a 44 nm thick n⁺-ZnSSe contact layer in this turn onthe p-GaAs substrate along with the above parameters. Production of theMQW will be later described.

An n-electrode of In is formed upon the n⁺-contact layer. The metallicIn n-electrode is an annular electrode or a small (dotted) roundelectrode having a wide aperture which allows incidence light to enter.A p-electrode of In or of Au—Zn—Pt is formed on the bottom of the p-GaAssubstrate. After the electrode formation, the GaAs wafer is cut anddivided into individual photodiode chips. A PD chip is upside downmounted upon a package for carrying the p-electrode in contact with apackage stem. The upward directed n-electrode is connected to a lead bya wire. A photodiode is made by fitting a lens and a cap to the package.

2. ZnSe/ZnTe-MQW Interposed Between GaAs and ZnSe (FIG. 9)

FIG. 9 shows an MQW which is interposed between a p-GaAs substrate and ap-ZnSe layer. All of the layers of the MQW are made by a molecular beamepitaxy method. Structure A and Structure B are shown as examples of theMQWs in FIG. 9. There are other available sets of layered structures.The number of layers is ten for both Structure A and Structure B. Othernumbers of layers (6, 8, 12, 14, . . . ) are also available. In theseexamples, impurity concentrations are N_(A)-N_(D)=3×10¹⁷ cm⁻³ in ZnSeand N_(A)-N_(D)=3×10¹⁹ cm⁻³ in ZnTe.

Structure A allocates a 2.1 nm thickness to all the p-ZnSe layers. Thethicknesses of the p-ZnTe layers are thicker near the GaAs substrate andthinner near the ZnSe buffer. Structure A gives the five ZnTe layersthicknesses of 1.5 nm, 1.2 nm, 0.9 nm, 0.6 nm and 0.3 nm respectivelyfrom GaAs to ZnTe. An average of the ZnTe layers is 0.9 nm. A sum of theZnTe thicknesses is 4.5 nm.

Structure B determines thicknesses of all the p-ZnSe films to be 2.1 nm.Thicknesses of the p-ZnTe films are narrower near the ZnSe interface andwider near the GaAs interface, similarly to Structure A. But,thicknesses of ZnTe films of Structure B are slightly different fromStructure A. The ZnTe films have 1.8 nm, 1.2 nm, 1.2 nm, 0.6 nm and 0.6nm respectively from GaAs to ZnTe. An average of thicknesses of ZnTefilms is 1.08 nm in Structure B. A total of the ZnTe thicknesses is 5.4nm.

Why does this invention provide photodiodes with such an MQW ofp-(ZnTe/ZnSe) between the p-GaAs substrate and the p-ZnSe layer? Thereason is now clarified. FIG. 10 shows a band structure of a jointbetween p-GaAs and p-ZnSe without MQW (Comparison Example 1). An uppersolid line curve is a conduction band Ec. A lower solid line curve is avalence band Ev. An intermediate horizontal direct dotted line isFermi's level E_(F). A difference between the conduction band and thevalence band is a forbidden band (or bandgap).

GaAs has a narrower bandgap of 1.42 eV. ZnSe has a wider bandgap of 2.68eV. There is a big bandgap difference of 1.26 eV. Since GaAs and ZnSeare p-type, Fermi's level lies close to the valence band. Fermi's levelsE_(F) should be equal on both sides of the GaAs/ZnSe junction. SinceGaAs and ZnSe are p-type, majority carriers are holes. Current iscarried by holes in the p-region. The holes run in the valence band.Attention should be paid to the valence band. KL denotes a valence bandof GaAs. MN denotes a valence band of ZnSe. A large gap LM occurs at the(GaAs/ZnSe) interface. Forward bias injects holes via a p-electrode intothe p-GaAs substrate. The holes should run in the p-GaAs upward, shouldpass the interface and should enter the p-ZnSe. However, the barrier LMstops the holes at the interface. The holes cannot overrun the highbarrier LM. This is the reason causing poor forward current property ofComparison Example 1 (without MQW).

FIG. 11 is a band diagram of a p-GaAs/MQW/ZnSe junction interposed witha p-(ZnSe/ZnTe) superlattice between p-GaAs and p-ZnSe (Embodiment 1).An upper zigzag solid line curve is a conduction band Ec. A lower zigzagsolid line curve is a valence band Ev. The intermediate zigzag partcorresponds to the MQW (ZnTe/ZnSe). A bandgap of GaAs is 1.42 eV. ZnSehas a bandgap of 2.68 eV. The difference of the bandgaps between ZnSeand GaAs is about 1.3 eV. The large band discontinuity prohibits holesfrom flowing the (GaAs/ZnSe)interface. ZnTe has a bandgap of 2.2 eVwhich is smaller than ZnSe. The MQW allows holes to pass the interfaceby tunnel conduction by making smoothly continuing hole levels in theinterface. The diagram of FIG. 11 includes three kinds of bandgaps ofGaAs, ZnTe and ZnSe.

A horizontal dotted line is Fermi's level which is common to all thefilms. Narrower ZnTe bandgaps and wider ZnSe bandgaps reciprocate in theMQW. This MQW has ten layers ofZnSe/ZnTe/ZnSe/ZnTe/ZnSe/ZnTe/ZnSe/ZnTe/ZnSe/ZnTe from GaAs to ZnSe.Thicknesses of ZnTe layers decrease from GaAs to ZnSe step by step. Inthe MQW, higher ZnSe films are called barriers and lower ZnTe films arecalled wells. As shown in FIG. 11, a bottom of the valence band of ZnSeis lower than a bottom of the valence band of ZnTe. But, a conductionband of ZnTe is higher than a conduction band of ZnSe in the ZnTe/ZnSejunction. Thus, the height of the ZnTe well is larger than the bandgapdifference between ZnSe and GaAs.

At an initial end, a hole injected from GaAs to ZnSe cannot exist in theZnSe, because the tops of valence bands are quite different between GaAsand ZnSe. The hole cannot overstep a high difference (LH). The hole,however, can pass the barrier by tunnel effect, because the ZnSe barrieris very thin (2.1 nm). Tunnel conduction allows the hole to move fromthe GaAs valence band to the ZnTe valence band. The ZnTe thin layermakes a narrow well in the valence band. The narrow ZnTe well makes aseries of quantized levels for holes. Allowable wavefunctions aredenoted by Ψ=b sin π(nx/d), where b is an amplitude, n is an integer andd is a width of the well. Schrödinger equation is −(h²/8m_(h) π)δ²Ψ/δx²=E Ψ, where h is Planck's constant and m_(h) is a mass of a hole.The minimum level is given by substituting n=1. A separation of thelevels is inversely-proportional to a square of the well width. Thelowest level E_(min) (zero-point vibration) in the first ZnTe well isdenoted byE _(min)=−h²/8 m_(h)d².   (1)

The origin is the top of the valence band of ZnTe. The minus “−” signshows a level of holes. In the representation, “d” is a width of theZnTe well (ZnTe thinness), “m_(h)” is an effective mass of a hole and“h” is Planck's constant. Namely, the level of a hole is lowered byE_(min) from the valence band top. A short line denotes the level ofE_(min). In the example, the MQW has ten films of ZnSe/ZnTe. Numerals 1,2, 3, . . . ,10 are allocated to the ten films from GaAs to ZnSe. 2nd,4th, 6th, 8th and 10th layers are ZnTe films. The thicknesses d of ZnTefilms decrease stepwise from GaAs to ZnSe for enlarging the minimum holelevel E_(min)=−h²/8m_(h)d². On the contrary, the thicknesses of ZnSefilms are all equal (2.1 nm). The thickness of a ZnSe film is determinedfor allowing holes to pass ZnSe layers by tunnel conduction.

A problem is how to design a set of ZnTe layers α, β, γ, δ and ε.Structure A which is shown in FIG. 9 as an example of a pertinent MQWgives thicknesses of 1.5 nm, 1.2 nm, 0.9 nm, 0.6 nm and 0.3 nm to thefive ZnTe layers α, β, γ, δ and ε. The minimum hole levels E_(min) arechanged in reverse proportion to the thicknesses. The minimum holeenergy E_(min) falls for the ZnTe layers α, β, γ, δ and ε. E_(min) is inproportion to −1/1.5² for ZnTe (α), −1/1.2² for ZnTe (β), 1/0.9² forZnTe (γ), 1/0:6² for ZnTe (δ), and −1/0.32 for ZnTe (ε). From the GaAsside to the ZnSe side, the minimum hole level E_(min) becomes deeper.The valence band rises from the GaAs side to the ZnSe side. Short linesin FIG. 11 denote E_(min) in the ZnTe layers α, β, γ, δ and ε. TheE_(min) levels are nearly equal to the common Fermi level E_(F). Holescan jump from a ZnTe E_(min) level to a next ZnTe E_(min) level bytunnel phenomenon induced by a strong reverse bias. Repetitions of thetunnel conduction carry holes from an initial point (L) of the GaAsvalence band to an end point (M) of the ZnTe valence band.

Namely, the ZnTe films are thicker on the GaAs side and thinner on theZnSe side. The quantized hole levels lower from the GaAs side to theZnSe side, which corresponds to rises of hole energy. The falling rateis designed to adjust to the rising curve of the valence band in atransition region from GaAs to ZnSe. The levels for holes become nearlycontinual. The continual levels enable holes to flow smoothly in theforward direction. What improves the forward current property fromComparison Example 1 to Embodiment 1 in FIG. 4 is the MQW of ZnTe/ZnSe.A gist of the present invention is the MQW. The MQW lowers forwardresistance without inducing absorption.

Embodiment 3 (On-p-GaAs ZnSe Type Avalanche Photodiode (APD): FIG. 19)

The present invention which makes a device by piling an nip-layeredstructure via an MQW on a p-GaAs substrate can be applied to avalanchephotodiodes (APD). APDs have two types of active layered structures. Oneis a simple pn-junction type for active layers which joins an n-layerdirectly to a p-layer. The other is a pin-junction type which joins ann-layer via an i-layer to a p-layer. The present invention can beapplied to both the pn- and pin-junction types. Reverse bias applied toan avalanche photodiode (APD) produces a strong electric field at apn-junction or a pin-junction. The reverse bias is determined to be alarge value slightly below a value inducing breakdown of the pn- orpin-junction. Incidence light produces pairs of electrons and holes indepletion layers. The strong electric field accelerates holes andelectrons (carriers). The carries collide with atoms and exciteelectrons or holes from the atoms. Multiple collisions increasecarriers. The increase of carriers induced by the strong electric fieldis called avalanche multiplication, which enable an APD to obtain alarge signal gain. The above two kinds of the active layers (pn-junctionor pin-junction) induce the avalanche multiplication. The pin-junctiontype interposing a thin i-layer is more suitable for realizing a stableAPD function. The interposition of an i-layer which has a good latticestructure without impurity enables the avalanche photodiode to suppressimpurity defects or other microscopic defects deriving from impurities.The intermediate i-layer has an advantage of making an excellent latticestructure, preventing strong electric field from inducing micro-plasmaat the junction.

An on-p-GaAs ZnSSe avalanche photodiode having a pin-junction isproduced and is examined.

Structure of a Pin-Junction Type APD Produced Upon a p-GaAs Substratefrom Top to Bottom (FIG. 19)

n-metallic electrode In dot-shaped φ = 0.8 mm n⁺-ZnS_(y)Se_(1−y) y =0.06 n = 1 × 10¹⁹ cm⁻³ thickness 20 nm n⁻-ZnS_(y)Se_(1−y) y = 0.06 n = 3× 10¹⁶ cm⁻³ thickness 700 nm i-ZnS_(y)Se_(1−y) y = 0.06 n < 1 × 10¹⁶cm⁻³ thickness 10 nm p-ZnS_(y)Se_(1−y) y = 0.06 p = 3 × 10¹⁷ cm⁻³thickness 450 nm p-ZnSe buffer p = 8 × 10¹⁷ cm⁻³ thickness 30 nmp-(ZnSe/ZnTe)^(m) superlattice(MQW) m = 5 (5 pairs of ZnSe/ZnTe layers)total thickness 12 nm p-GaAs substrate p = 2 × 10¹⁹ cm⁻³ substratep-metallic electrode In—Au thickness 300 μm

The above layered structure is produced upon a p-GaAs wafer by an MBEmethod. The APD wafer is cut into plenty of tiny chips of a I mm square.The PD chips are shaped into a mesa-shape by wet-etching, as shown inFIG. 19. An APD chip is fitted upon a mount of a package with leads. Thetop electrode is bonded to a lead by a wire. The APD chip is sealed by acap with an aperture.

FIG. 20 shows measured temperature variations of avalanche breakdownproperties of the APD of Embodiment 3. The ordinate is electric current(mA). The abscissa is reverse bias voltage (V). Parameters aretemperatures (K) which are indicated below the curves. In a hightemperature range between 300K and 320K, breakdown voltages Vb are from−29V to −30 V. In a low temperature range, the breakdown voltagedecreases to −25.5V. Conspicuous temperature dependence of the breakdownvoltage is a result of a temperature dependence of carrier mobility.This result indicates occurrence of an intrinsic avalanche breakdown inthe APD.

FIG. 21 shows variations of photocurrent gains (M) (multiplication rate)as a function of an electric field (in proportion to reverse bias) belowthe breakdown voltage at room temperature. The measurement of thephotocurrent gains is proceeded by irradiating the APD by a He—Cd laserof a 442 nm wavelength. The APD has no antireflection film. At roomtemperature, the largest multiplication (gain) of 8.9 is obtained at areverse bias of −29V. The large gain is an advantage of the APD. Anotheradvantage is a small breakdown voltage (−29V to −30V), which is farsmaller than the avalanche breakdown voltages (−50V to −80V) of siliconphotodiodes or germanium photodiodes. The small breakdown voltage iseffective for heightening stability and reliability of the APD of thepresent invention.

A preliminary aging examination of 5000 hours confirmed excellentstability of the APD of the present invention. The aging examinationproved that no increment (DC-increase or abrupt hike) of dark currentoccurs under a reverse bias from −25V to −29.3V.

Embodiment 3 of an APD made of ZnSSe (np- or nip-) layers formed on ap-GaAs substrate exhibits high sensitivity for blue light. The presentinvention includes an APD which is constructed by ZnMgSSe (np- or nip-)layers formed on p-GaAs. The ZnMgSSe layered APD has a wide sensitivityrange of blue, violet and ultraviolet rays. An addition of anantireflection film or a protective film on the top aperture enablesthis invention to produce higher sensitive, more stable and morepractical avalanche photodiodes for a short wavelength range from blueto ultraviolet.

Embodiment 4 (On-p-GaAs ZnSe Type Avalanche Photodiode (APD): FIG. 22)

Another on-p-GaAs ZnSSe avalanche photodiode having the followingstructure is produced and is examined.

[Layered Structure (From Top to Bottom) n-metallic electrode Indot-shaped φ = 0.8 mm n⁺-ZnS_(y)Se_(1−y) y = 0.06 n = 1 × 10¹⁹ cm⁻³thickness 10 nm n⁻-ZnS_(y)Se_(1−y) y = 0.06 n = 3 × 10¹⁶ cm⁻³ thickness700 nm i-ZnS_(y)Se_(1−y) y = 0.06 n < 1 × 10⁶ cm⁻³ thickness 10 nmp-ZnS_(y)Se_(1−y) y = 0.06 p = 3 × 10¹⁷ cm⁻³ thickness 450 nm p-ZnSebuffer p = 8 × 10¹⁷ cm⁻³ thickness 30 nm p-(ZnSe/ZnTe)^(m) superlattice(MQW) m = 5 (5 pairs of ZnSe/ZnTe layers) total thickness 12 nm p-GaAssubstrate p = 2 × 10¹⁹ cm⁻³ substrate p-metallic electrode In—Authickness 300 μm

The above layered structure is produced upon a p-GaAs wafer by an MBEmethod. The APD wafer is cut into plenty of tiny chips of a 1 mm square.The PD chips are shaped into a mesa-shape by wet-etching. An APD chip isfitted upon a mount of a package with leads. The top electrode is bondedto a lead by a wire. The APD chip is sealed by a cap with an aperture.

Embodiment 4 has a 10 nm thick top n-ZnSSe layer which is thinner thanEmbodiment 3. A low resistance ohmic n-electrode can be formed upon sucha thick n-layer. Thinning of the top n-ZnSSe layer reduces lightabsorption and enhances quantum efficiency. FIG. 22 is a graph showing aresult of measurement of quantum efficiency as a function of input lightwavelengths at room temperature. The abscissa is a wavelength (nm) ofinput light. The ordinate is external quantum efficiency (%). Parametersare reverse biases of Vb=0V, −30V, −32V, −34V and −35V. An increase ofthe reverse bias raises external quantum efficiency. Embodiment 3 hasapplied reverse bias to −29V under He—Cd laser input λ=442 nm. But,Embodiment 4 applies stronger reverse bias to −35V.

“E₀” shows the bandgap energy of ZnSSe corresponding to a wavelength of457 nm (λ=1239.8/Eg). Sensitivity falls for wavelengths longer than thebandgap wavelength. The APD has nearly flat sensitivity for wavelengths(from 300 nm to 450 nm) shorter than the bandgap wavelength.

External quantum efficiency is about 25% for a 0V reverse bias at a 300nm wavelength. The quantum efficiency rises to 40% at 400 nm and to 60%at 450 nm for the 0 bias.

In the case of a reverse bias of −30V, external quantum efficiency is60% at 300 nm, 100% at 400 nm and 110% at 450 nm.

A −32V reverse bias enhances quantum efficiency up to 80% at 300 nm,130% at 400 nm and 140% at 450 nm.

In the case of a reverse bias of −34V, external quantum efficiency is150% at 300 nm, 230% at 400 nm and 220% at 450 nm.

The strongest −35V reverse bias exhibits 260% at 300 nm, 330% at 400 nmand 300% at 450 nm. The breakdown voltage Vb of the present inventionAPD is equal to or less than 35V (Vb≧−35V).

When the reverse bias is raised, the quantum efficiency of thejust-below bandgap region (457 nm) is surpassed by a shorter wavelengthregion of energy E₀+Δ, which corresponds to 395 nm. The 360 nmwavelength region shows 370% of external quantum efficiency.

The result shows that the APD exceeds the pin-PD of FIG. 13 in thesensitivity for the short wavelength region near 360 nm. In the case ofthe pin-PD of FIG. 13, the external quantum efficiency represented by aunit of mA/W which is a quotient of dividing photocurrent (mA) by lightpower (W) draws a rightward rising curve which takes a maximum near thebandgap. Energy of light increase in proportion to an inverse (1/λ) of awavelength. But, photocurrent induced by the incidence light is nearlyconstant. Then, the quantum efficiency rises as the wavelength increasesin the range below the bandgap wavelength.

The avalanche photodiode of Embodiment 4 has an advantage of highquantum efficiency for the light of wavelengths shorter than the bandgapwavelength.

E₀+Δ_(s) (λ=395 nm) corresponds to the bandgap of forbidden band splitby the spin-orbit interaction (LS coupling).

Blue, violet and ultraviolet rays below 457 nm are absorbed by then⁻-ZnMgSSe region and are converted to pairs of electrons and holes.Holes, minority carriers, are accelerated by a strong electric fieldformed in the n⁻-ZnMgSSe layer. Collision of high speed holes with atomsyields new carriers. The carriers further generate new carriers.Avalanche amplification takes place. Light beams of wavelengths of 395nm or less than 395 nm produce holes in the lower branch of the valenceband slit by the LS coupling. The mass of the hole has a small effectivemass, which increases a multiplication rate. The small mass holes liftthe efficiency curve near the peak point E₀+Δ_(s) (=395 nm). The LScoupling which makes the double valence band having light mass holes hasthe effect of enhancing the quantum efficiency curve at 395 nm and ofmaking a near-flat sensitivity property, which is a favorable propertyfor sensing ultraviolet rays whose wavelengths are less than 380 nm.Such an enhancement property cannot be expected for GaN type photodiodeswhich are also candidates for ultraviolet detection.

The ZnMgSSe nip-PD and APD of the present invention have been describedabout production methods and sensitivity by referring tothe-embodiments. Fundamental properties of the nip-PD and APD of thepresent invention are as follows.

-   -   (1) A reduction of dark current under a reverse bias down to a        level of half of dark current of current Si-PDs which have been        the least dark current among various kinds of practical PDs.    -   (2) A large photovoltage (1.5V-1.7V) which is about five times        as large as a photovoltage of Si-PDs.

The excellent properties derives from the fundamental structure offorming an MQW on a p-GaAs substrate and forming a ZnMgSSe nip- ornp-layers on the MQW.

1-10. (canceled)
 11. An on-p-GaAs substrate Zn_(1-x)Mg_(x)S_(y)Se_(1-y)avalanche photodiode for inducing avalanche amplification by a strongelectric field formed by applying a reverse bias below a breakdownvoltage, comprising: a p-GaAs single crystal substrate having a topsurface and a bottom surface; a p-(ZnSe/ZnTe)^(m) (m: integer denoting anumber of pair layers) superlattice which is made by piling p-ZnSe thinfilms and p-ZnTe thin films reciprocally for changing bandgaps stepwiseand is epitaxially grown on the top surface of the p-GaAs substrate; ap-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) (0≦x≦0.8, 0≦y≦0.8) layer epitaxially grownon the p-(ZnSe/ZnTe)^(m) superlattice or via a p-ZnSe buffer layer uponthe p-(ZnSe/ZnTe)^(m) superlattice; a lower-dopedn⁻-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) (0≦x≦0.8, 0≦y≦0.8) layer epitaxiallygrown on the p-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) layer; a higher-dopedn⁺-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) (0≦x≦0.8, 0≦y≦0.8) layer epitaxiallygrown on the lower-doped n^(−-Zn) _(1-x)Mg_(x)S_(y)Se_(1-y) layer; ametallic n-electrode which is formed upon a part of the higher-dopedn⁺-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) layer and has a top aperture for allowingincidence light to enter; and a metallic p-electrode formed on thebottom surface of the p-GaAs substrate.
 12. The on-p-GaAs substrateZn_(1-x)Mg_(x)S_(y)Se_(1-y) avalanche photodiode according to claim 11,wherein a p-ZnSe buffer layer is interposed between thep-(ZnSe/ZnTe)^(m) superlattice and the p-Zn_(1-x)Mg_(x)S_(y)Se_(1-y)layer.
 13. The on-p-GaAs substrate Zn_(1-x)Mg_(x)S_(y)Se_(1-y) avalanchephotodiode according to claim 11, wherein ani-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) (0≦x≦0.8, 0≦y≦0.8) layer is interposedbetween the p-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) layer and then⁻-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) layer.
 14. The on-p-GaAs substrateZn_(1-x)Mg_(x)S_(y)Se_(1-y) avalanche photodiode according to claim 11,wherein the n⁺-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) layer has a bandgap En⁺ whichis equal to or higher than a bandgap En⁻ of then⁻-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) layer (En⁺≧En⁻.
 15. The on-p-GaAssubstrate Zn_(1-x)Mg_(x)S_(y)Se_(1-y) avalanche photodiode according toclaim 14, wherein the n⁻-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) layer is ann⁻-ZnS_(y)Se_(1-y) layer including no Mg (x=0) and then⁺-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) layer is either ann⁺-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) layer including Mg (x≠0) or ann⁺-ZnS_(y)Se_(1-y) layer including no Mg (x=0).
 16. The on-p-GaAssubstrate Zn_(1-x)Mg_(x)S_(y)Se_(1-y) avalanche photodiode according toclaim 14, wherein the n⁻-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) layer is an n⁻-ZnSelayer including neither Mg nor S (x=0, y=0) and then⁺-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) layer is either an n⁺-ZnS_(y)Se_(1-y)layer including no Mg (x=0, y≠0) or an n⁺-ZnSe layer including neitherMg nor S (x=0, y=0).
 17. The on-p-GaAs substrateZn_(1-x)Mg_(x)S_(y)Se_(1-y) avalanche photodiode according to claim 11,wherein the top aperture on the n⁺-Zn_(1-x)Mg_(x)S_(y)Se_(1-y) layerwhich receives incidence light is coated with a mask made of Al₂O₃,SiO₂, TiO₂, La₂O₃ or MgF₂ for antireflection and protection.
 18. Theon-p-GaAs substrate Zn_(1-x)Mg_(x)S_(y)Se_(1-y) avalanche photodiodeaccording to claim 11, wherein external quantum efficiency is more than100% for light wavelengths between 300 nm and 450 nm.
 19. The on-p-GaAssubstrate Zn_(1-x)Mg_(x)S_(y)Se_(1-y) avalanche photodiode according toclaim 11, wherein external quantum efficiency is more than 200% for alight wavelength of 400 nm.
 20. The on-p-GaAs substrateZn_(1-x)Mg_(x)S_(y)Se_(1-y) avalanche photodiode according to claim 11,wherein external quantum efficiency is enhanced by a spin-orbitinteraction at a wavelength of 395 nm and sensitivity is nearly flatfrom 350 nm to 430 nm.